Method of forming multilayer coating films and multilayer coating films

ABSTRACT

It is an object of the present invention to provide a method of forming a multilayer coating film by which the multilayer coating film comparable in shock resistance, in particular chipping resistance, to the conventional three-coat films and resistant to yellowing and having a good appearance can be formed in the three-wet coating system which is intended for coating process curtailment, cost reduction and environmental impact reduction.  
     The present invention provides a method of forming a multilayer coating film comprising the step (I) of coating an article to be coated with an electrodeposition coating followed by curing by heating to form an electrodeposited coating film, the step (II) of applying a water-borne intermediate coating onto said electrodeposited coating film to form an uncured intermediate coating film, the step (III) of applying a water-borne base coating onto said intermediate coating film to form an uncured base coating film, the step (IV) of applying a clear coating onto said base coating film to form an uncured clear coating film and the step (V) of curing said intermediate coating film, said base coating film and said clear coating film simultaneously by heating to thereby obtain a multilayer coating film.

TECHNICAL FIELD

[0001] The present invention relates to a method of forming a multilayercoating film utilizing a three-wet coating system which comprisesapplying a water-borne intermediate coating, a water-borne base coatingand a clear coating, in that order, onto an electrodeposited coatingfilm formed on an article to be coated in the wet-on-wet manner andcuring simultaneously by heating. More specifically, it relates to amethod of forming a multilayer coating film by which the multilayercoating film excellent in chipping resistance and showing no tendencytoward yellowing can be obtained, and to a multilayer coating filmobtained by this method.

BACKGROUND ART

[0002] In recent years it has been urgently demanded that the coatingprocess be curtailed in the field of coatings, in particular in thefield of automobile coatings, so that the problems of saving resources,reducing costs and minimizing environmental impacts (VOC and HAPs, etc.)may be solved. In the conventional procedure for finishing coatingautomobiles, the three-coat three-bake coating technique has been used,namely the electrodeposited coating film, intermediate coating film andtop coating film are baked each time after application of eachcorresponding coating. In recent years, however, it has been demandedthat those coating film performance characteristics which can beexhibited by the three-coat films obtained by the conventionalthree-coat three-bake coating technique be acquired by employing thethree-wet coating system according to which the three coating steps,namely intermediate coating, base coating and clear coating, are carriedout in the wet-on-wet manner following the step of electrodepositioncoating and baking of the electrodeposited coating film and theresulting wet coating films are baked simultaneously, while therebyreducing the number of baking process steps.

[0003] Among the coating film performance characteristics referred toabove, the shock resistance, in particular the so-called chippingresistance to collision of pebbles or like obstacles with the car bodyduring running, can be secured by the conventional three-coat three-bakecoating technique, for example by providing a particular intermediatecoating film having chipping resistance. In the three-wet coating systemmentioned above, however, the conventional intermediate coatings cannotbe used since the coating films obtained tend to be impaired inappearance due to such troubles as blurring or layer inversion. Thecoating films obtained by the three-wet coating system aredisadvantageous in that they are low in shock resistance and chippingresistance.

[0004] Japanese Kokai Publication Sho-62-65765 discloses a techniqueaccording to which a resin layer capable of absorbing a shock to coatingfilms (the so-called anti-chipping primer layer) is provided duringmultilayer film formation, in particular between the electrodepositedcoating film and intermediate coating film. However, furtherincorporation of such a step in the car body coating process is againstthe market needs for curtailment of the process and for cost savingmentioned above.

[0005] On the other hand, water-borne coatings have attracted attentionin recent years in the field of coatings, in particular in the field ofautomobile coatings, since they can contribute to reductions inenvironmental impacts (e.g. VOC). Water-borne coatings are generallyprepared by solubilizing, dispersing or emulsifying a coatingfilm-forming resin in water and/or a hydrophilic solvent such as analcohol. When the coating film-forming resin has an anionic functionalgroup, such as carboxyl group, introduced therein, a basic substancesuch as an amine is used as the counter ion and, therefore, the use ofsuch water-borne coatings in the above-mentioned three-wet coatingsystem produces the problem of yellowing of the coating films obtained.

[0006] Accordingly, it is an object of the present invention to providea method of forming a multilayer coating film by which the multilayercoating film comparable in shock resistance, in particular chippingresistance, to the conventional three-coat films and resistant toyellowing and having a good appearance can be formed in the three-wetcoating system which is intended for coating process curtailment, costreduction and environmental impact reduction.

SUMMARY OF THE INVENTION

[0007] The present invention provides a method of forming a multilayercoating film comprising

[0008] the step (I) of coating an article to be coated with anelectrodeposition coating followed by curing by heating to form anelectrodeposited coating film,

[0009] the step (II) of applying a water-borne intermediate coating ontosaid electrodeposited coating film to form an uncured intermediatecoating film,

[0010] the step (III) of applying a water-borne base coating onto saidintermediate coating film to form an uncured base coating film,

[0011] the step (IV) of applying a clear coating onto said base coatingfilm to form an uncured clear coating film and

[0012] the step (V) of curing said intermediate coating film, said basecoating film and said clear coating film simultaneously by heating tothereby obtain a multilayer coating film,

[0013] wherein said electrodeposition coating contains a particle Acontaining a resin (a) whose solubility parameter is δa as well as aparticle B containing a curing agent and a resin (b) whose solubilityparameter is δb and satisfies that

[0014] (1) the value of (δb -δa) is not less than 1.0,

[0015] (2) as regards the electrodeposited coating film formed from saidelectrodeposition coating, the resin film formed from said particle Ashows a dynamic glass transition temperature of −110 to 10° C. and

[0016] the coating film obtained by film formation from said particle Aalone shows an elongation percentage of not less than 200% and

[0017] (3) as regards the electrodeposited coating film formed from saidelectrodeposition coating, the resin film formed from said particle Bshows a dynamic glass transition temperature of 60 to 150° C., and

[0018] wherein the total amount of volatile basic substance in saiduncured intermediate coating film and said uncured base coating filmprior to carrying out the step (V) is not more than 7×10⁻⁶ mmol percoating film unit area (1 mm²).

[0019] The invention is also directed to a multilayer coating film whichis obtained by the above-mentioned method of forming a multilayercoating film.

DISCLOSURE OF THE INVENTION

[0020] The method of forming a multilayer coating film according to thepresent invention comprises the step (I) of coating an article to becoated with an electrodeposition coating followed by curing by heatingto form an electrodeposited coating film, the step (II) of applying awater-borne intermediate coating onto the above electrodeposited coatingfilm to form an uncured intermediate coating film, the step (III) ofapplying a water-borne base coating onto the above intermediate coatingfilm to form an uncured base coating film, the step (IV) of applying aclear coating on to the above base coating film to form an uncured clearcoating film and the step (V) of curing the above intermediate coatingfilm, the above base coating film and the above clear coating filmsimultaneously by heating to thereby obtain a multilayer coating film.

[0021] Step (I)

[0022] In the method of forming a multilayer coating film according tothe invention, the above step (1) comprises applying anelectrodeposition coating on an article to be coated, followed by curingby heating to form an electrodeposited coating film.

[0023] Electrodeposition Coating

[0024] The above electrodeposition coating contains a particle Acontaining a resin (a) whose solubility parameter is δa as well as aparticle B containing a curing agent and a resin (b) whose solubilityparameter is δb and in which

[0025] (1) the value of (δb -δa) is not less than 1.0,

[0026] (2) as regards the electrodeposited coating film formed from theabove electrodeposition coating, the resin film formed from the aboveparticle A shows a dynamic glass transition temperature of −110 to 10°C.,

[0027] and the coating film obtained by film formation from the aboveparticle A alone shows an elongation percentage of not less than 200%,and

[0028] (3) as regards the electrodeposited coating film formed from theabove electrodeposition coating, the resin film formed from the aboveparticle B shows a dynamic glass transition temperature of 60 to 150° C.

[0029] The above electrodeposition coating, in which two resincomponents incompatible with each other are used, can form anelectrodeposited coating film having a multilayer structure so that aresin layer having corrosion resistance may be formed on the side incontact with the article to be coated and a resin layer having shockresistance (chipping resistance) on the side in contact with air tothereby attain high levels of corrosion resistance and shock resistancesimultaneously.

[0030] The above electrodeposition coating contains a particle Acontaining a resin (a) whose solubility parameter is δa as well as aparticle B containing a curing agent and a resin (b) whose solubilityparameter is δb. In the present specification, it is meant thatparticles A and particles B are prepared in the form of separateemulsions and are mixed up in preparing the electrodeposition coatingbut occur as separate particles in the coating without being fusedtogether.

[0031] In the above electrodeposition coating, the difference (δb-δa)between the solubility parameter δa of the above resin (a) and thesolubility parameter δb of the above resin (b) is not less than 1.0. Byselecting two incompatible or hardly compatible resin components suchthat the said value of (δb-δa) is not less than 1.0, it becomes possibleto form electrodeposited coating films having a multilayer structure.

[0032] It is generally considered that when the difference in solubilityparameter between resins is 0.5 or more, the compatibility between themis lost and the coating films show a structure reflecting phaseseparation. In the case of the above electrodeposition coating, however,it is necessary that a coating film structure showing distinct layerseparation be formed and, therefore, it is necessary that the differencein solubility parameter be at least 1.0 or more. If the difference isless than 1.0, any coating film structure showing distinct layerseparation will not be formed in the step of electrodeposition coating,hence the levels of shock resistance, in particular chipping resistance,and corrosion resistance which are attainable simultaneously will beunsatisfactory.

[0033] The above-mentioned solubility parameter δ is generally called SP(solubility parameter) and is an index of the hydrophilicity orhydrophobicity of a resin and serves as an important index in estimatingthe compatibility between resins. The above solubility parameter can beexpressed numerically based on the hitherto-known technique ofturbidimetry measurement by skilled in the art (K. W. Suh, D. H. Clarke,J. Polymer. Sci., A-1, 5, 1671 (1967)).

[0034] Among the above resins (a) and (b), the one having a highersolubility parameter, namely resin (b), is higher in affinity for theelectroconductive substrate surface such as a metal, which is higher insurface polarity, so that the electrodeposited coating film formed fromparticles B containing the resin (b) is formed on the side contactingthe conductive substrate made of a metal or like material in the step ofcuring by heating. On the other hand, the particles A containing resin(a) migrate toward the air-contacting side to form another resin layer.Thus, the difference in solubility parameter between the two resins isconsidered to serve as a driving force for causing resin layerseparation.

[0035] The state of the above resin layer separation can be confirmed byvisual observation of a section of the electrodeposited coating film bymeans of a video microscope or observation under a scanning electronmicroscope (SEM observation). For identifying the resin componentsconstituting the respective resin layers, a total reflection Fouriertransform infrared photometer (FTIR-ATR), for instance, can be used.

[0036] Of the electrodeposited coating film formed from the aboveelectrodeposition coating, the resin film formed from the particles Acontaining resin (a) mentioned above shows a dynamic glass transitiontemperature of −110 to 10° C. If it is above 10° C., the coating filmobtained from particles A will be poor in flexibility or shockresistance. If it is below −110° C., it is difficult in practice toprepare. Preferred is −100to −30° C.

[0037] The above-mentioned dynamic glass transition temperature can bemeasured by using a dynamic viscoelasticity measuring apparatus such asRheovibron (product of Orientec) or a Rheometrix dynamic analyzer(product of Rheometrix) following electrodeposition of a substrate withthe above electrodeposition coating, curing and peeling of theelectrodeposited coating film using mercury.

[0038] As regards the particles A containing resin (a) mentioned above,the coating film obtained by film formation from the particles A aloneshows an elongation percentage of not less than 200%. When it is lessthan 200%, the coating films obtained become poor in elasticity.Preferably, it is not less than 500%. The above elongation percentagecan be determined according to JIS K 6301.

[0039] The above resin (a) is not particularly restricted on conditionthat the above characteristics requirements are satisfied. Thus, itincludes, for example, homopolymers of a conjugated diene monomer suchas butadiene, isoprene or chloroprene, or random or block copolymers ofa conjugated diene monomer and such a monomer as ethylene, propylene,ethylidene, norbornene, dicyclopentadiene, 1,4-hexadiene, vinyl acetate,vinyl chloride, styrene, acrylonitrile, isobutylene or (meth) acrylicacid (ester); polyurethane-based thermoplastic elastomers synthesized bypolyaddition reaction of a diisocyanate and a diol; polyester-basedthermoplastic elastomers synthesized by transesterification andpolycondensation reactions using dimethyl terephthalate, 1,4-butanediol,poly(tetramethylene) glycol, etc. as raw materials; and polyamide-basedthermoplastic elastomers synthesized by transesterification andpolycondensation reactions using a lactam, a dicarboxylic acid andpolyether diol as raw materials.

[0040] In the above electrodeposition coating, the above resin (a) ispreferably an elastomer (rubber) produced by polymerizing a monomercomponent comprising at least 50% by weight of a conjugated dienemonomerin view of the possible shock resistance manifestation level, economy(cost) and general versatility. If the diene content is less than 50% byweight, it will become difficult to constitute a resin layer showing theabove glass transition temperature and elongation percentage in the stepof coating film formation and, as a result, the shock resistance andchipping resistance will decrease. An elastomer produced by polymerizinga monomer component comprising not less than 60% by weight of aconjugated diene monomer is more preferred and not less than 65% isstill more preferred.

[0041] The molecule of the above resin (a) may contain, within themolecular structure and/or at a terminus thereof, a reactive group orpolar group such as a hydroxyl, amino, vinyl, carboxyl, urethane or ureagroup. The above reactive group or polar group can be introduced bycopolymerizing a monomer component comprising a reactive or polargroup-containing monomer in the step of preparing a resin (a) or by amethod known in the art into a resin (a) obtained by copolymerization.

[0042] The above copolymerization is preferably carried out in thepresence of a radical polymerization initiator. As the radicalpolymerization initiator, there may be mentioned, for example, azoinitiators such as 2,2′-azobisisobutyronitrile and2,2′-azobis(2,4-dimethylvaleronitrile); and peroxide initiators such asbenzoyl peroxide, lauryl peroxide and tert-butyl peroctoate. Theseinitiators are used in an amount of 0.2 to 10 parts byweight, preferably0.5to 5parts by weight, per 100 parts by weight of polymerizablemonomers in total.

[0043] When the above resin (a) is an oligomer (liquid rubber) having anumber average molecular weight less than 10, 000, the resin is highlysticky and, as such, has low shock resistance performancecharacteristics, so that it is necessary to subject the same to curingreaction in the step of coating film formation in order to express thedesired coating film performance characteristics, such as shockresistance. In that case, it is preferred that hydroxyl groups arecontained so as to give a hydroxyl value within the range of 20 to 200.At a hydroxyl value less than 20, the coating film may fail to be curedto a sufficient extent, hence fail to express satisfactory rubberperformance characteristics such as sufficient elongation percentage. Ifit is above 200, excess hydroxyl groups remain in the coating film aftercuring, hence the water resistance may decrease. Furthermore, thehardness of the cured coating film increases, leading to failure toexpress a sufficient level of elongation percentage.

[0044] When the above resin (a) has a high number average molecularweight not less than 10, 000, if it has little stickiness without curingand shows sufficient shock resistance performance characteristics, acuring reaction is not particularly required in the step of coating filmformation. In such cases, it is not necessary to provide the resinstructure with reactive groups or polar groups in advance.

[0045] The molecular weight of the above resin (a) is not particularlyrestricted but preferably is within the range of 1,000 to 200,000 interms of number average molecular weight. If it is less than 1,000, itwill be difficult to obtain coating films showing an elongationpercentage exceeding 200% even when the crosslinking reaction iseffectively carried out in the step of coating film formation. If itexceeds 200,000, the resin solution will become highly viscous so thatnot only the handling of the obtained resin in such operations asemulsification/dispersion becomes difficult but also the film appearanceof the electrodeposited coating films obtained may be markedly impaired.Further, in some instances, it becomes difficult, due to the highviscosity, to secure the layer separation in the step of baking ofcoating films.

[0046] The above resin (a), when emulsified and dispersed, independentlyof the resin (b), in an aqueous medium, constitutes the particles A.

[0047] The above resin (a) may introduce a cationic group such as anamino group among the above-mentioned reactive or polar groups by aurethane-forming reaction or the like to use the resulting products asthey are or self-emulsifiable and dispersible in an aqueous medium byusing a neutralizing agent. Or, it is also possible to emulsify anddisperse it in an aqueous medium by separately applying a cationicemulsifier. On that occasion, if necessary, an appropriate amount of acuring agent, for instance, may be added to the resin andemulsified/dispersed together. As the above neutralizing agent, theremay be mentioned inorganic acids such as hydrochloric acid, nitric acidandphosphoric acid; andorganic acids such as formic acid, acetic acid,lactic acid, sulfamic acid and acetylglycine acid.

[0048] In the above electrodeposition coating, the resin (a) ispreferably emulsified/dispersed in an aqueous medium using a cationicemulsifier, since the hydrophobicity of the particles A containing resin(a) as a whole then increases and a multilayer structure with a distinctlayer separation can be obtained.

[0049] The above cationic emulsifier is not particularly restricted butmay be anyone having a cationic group. Preferably, it has a numberaverage molecular weight of 1,000 to 200,000. If it is less than 1,000,the water resistance of coating films may be adversely affected. If itexceeds 200, 000, the system will become highly viscous in the step ofbaking of coating films, so that the layer separation may possibly beinhibited.

[0050] For securing the emulsifiability/dispersibility of the aboveresin (a), the cationic group content of the above cationic emulsifier,namely the content of the amino group, ammonium salt group or sulfoniumsalt group in the emulsifier, is preferably about 30 to 150 as expressedin terms of amine value equivalent. If it is less than 30, the effect ofemulsifying/dispersing the resin (a) will be poor. If it exceeds 150,the water resistance and other properties of coating films may beadversely affected.

[0051] The above cationic emulsifier is incorporated preferably in anamount within the range of 10 to 50% by weight on the solid basisrelative to 100 parts by weight of the resin (a) on the solid basis. Ifthe amount is less than 10% by weight, the dispersion stability of theemulsion will become poor and if it exceeds 50% by weight, not only thewater resistance of coating films will deteriorate but also it willbecome difficult for such characteristic features owing to resin (a) asshock resistance to be fully expressed.

[0052] The above cationic emulsifier can be prepared by providing themain chain of a resin with a cationic group by carrying out anappropriate reaction by a method known in the art. The resin skeleton ofthe above cationic emulsifier is not particularly restricted but may bean acrylic resin, epoxy resin, liquid rubber (elastomer), polyurethaneor polyether, or a modified resin based thereon, for instance.

[0053] Those having the above-mentioned acrylic resin as the resinskeleton can be synthesized, for example, by subjecting an acryliccopolymer containing a plurality of epoxy groups within the molecule andan amine to ring opening addition reaction. Thus, a cationic acrylicresin can be obtained by copolymerizing an epoxy group-containingacrylic monomer such as glycidyl (meth) acrylate with another monomerand subjecting all the epoxy groups of the resulting epoxygroup-containing acrylic resin to ring opening by reacting with anamine.

[0054] The above amine is not particularly restricted but mention may bemade of, for example, primary, secondary and tertiary amine acid saltssuch as butylamine, octylamine, diethylamine, dibutylamine,methylbutylamine, monoethanolamine, diethanolamine,N-methylethanolamine, triethylamineacidsalts andN,N-dimethylethanolamine acid salts. Ketimine-blocked primary aminogroup-containing secondary amines, such as aminoethylethanolamine methylisobutyl ketimine, may also be used. For causing all epoxy rings toopen, it is necessary that these amines be reacted with the epoxy ringsat least in an equivalent amount.

[0055] The above cationic acrylic resin can also be obtained by a directsynthetic method comprising copolymerizing an amino group-containingacrylic monomer with another monomer. The above amino group-containingacrylic monomer includes N,N-dimethylaminoethyl(meth)acrylate andN,N-di-tert-butylaminoethyl(meth)acrylate, etc.

[0056] The other monomer to be copolymerized with the above epoxygroup-containing acrylic monomer or amino group-containing acrylicmonomer is not particularly restricted but includes, for example,hydroxyl group-containing acrylic monomers, other acrylic monomers andnon-acrylic monomers. The hydroxyl group-containing acrylic monomersmentioned above can serve to increase the reactivity in curing, henceare preferably used.

[0057] When the resin skeleton is the above-mentioned epoxyresin, acationic group can be introduced therein by modifying the epoxy groupsin the resin in the same manner as mentioned above.

[0058] When the resin skeleton is the above-mentioned liquid rubber(elastomer), polyurethane orpolyether, a cationic group can beintroduced therein by subjecting hydroxyl, carboxyl, epoxy or likegroups occurring at the molecular terminus and/or in the middle of themolecular structure to urethane formation reaction or addition reactionof an amine.

[0059] The cationic emulsifier mentioned above may have a primaryhydroxyl group introduced therein for providing the reactivity in curingor a long-chain alkyl group, such as stearyl, dodecyl or octyl groups,introduced therein for improving the ability to be adsorbed on the aboveresin (a). These can be introduced by reacting functional groups in themain chain with a hydroxyl group-containing secondary amine or along-chain alkyl group-containing secondary amine, or bycopolymerization using a monomer having such a group.

[0060] In the above cationic emulsifier, the above cationic group playsa roll as a hydrophilic group. Furthermore, the mutual adsorption effectwith the above resin (a) can be secured by means of the flexible mainchain moiety and hydrophobic moieties such as alkyl groups and benzenestructures occurring in the cationic emulsifier. The above cationicemulsifier can be dissolved or dispersed as such in an aqueous medium.

[0061] The above-mentioned particles Amay contain a curing agent.

[0062] The above curing agent includes isocyanate curing agents,melamine curing agents and amide curing agents. Preferred are blockedpolyisocyanates.

[0063] As examples of the polyisocyanates serving as raw materials forthe above blocked polyisocyanates, there may be mentioned aliphaticdiisocyanates such as hexamethylene diisocyanate, tetramethylenediisocyanate and trimethylhexamethylene diisocyanate; alicyclicpolyisocyanates such as isophorone-diisocyanate and4,4′-methylenebis(cyclohexyl isocyanate); aromatic diisocyanates such as4,4′-diphenylmethanediisocyanate, tolylene diisocyanate and xylylenediisocyanate, and polymers derived from these. The above-mentionedblocked polyisocyanates can be obtained by blocking these with anappropriate blocking agent.

[0064] Examples of the blocking agent are monohydric alkyl (or aromatic)alcohols such as n-butanol, n-hexyl alcohol, 2-ethylhexanol, laurylalcohol, phenolcarbinol and methylphenylcarbinol; cellosolves such asethylene glycol monohexyl ether and ethylene glycol mono-2-ethylhexylether; phenols such as phenol, p-tert-butylphenol and cresol; oximessuch as dimethyl ketoxime, methyl ethyl ketoxime, methyl isobutylketoxime, methyl amyl ketomixe and cyclohexanone oxime; and lactams suchas ε-caprolactam and γ-butyrolactam. Oximes and lactams are preferredfrom the viewpoint of resin curability since these dissociate at lowtemperature.

[0065] The percentage of blocking with the above blocking agent ispreferably 100% so that the storage stability of the coating can besecured.

[0066] The above polyisocyanates and blocking agents may respectively beused singly or two or more may be used in combination. A plurality ofthe resulting blocked polyisocyanates may also be used in combinationfor the purpose of adjusting the coating film physical properties or thedegree of curing.

[0067] In curing the resin layer composed of the particles A containingthe resin (a) mentioned above in the above step (I), it is preferredthat the solubility parameter (δi) of at least one curing agent such asmentioned above have a value between the solubility parameter δa ofresin (a) and the solubility parameter δb of resin (b), namely satisfythe relation δa<δi <δb. This makes it possible for the blockedpolyisocyanate to be distributed and dissolved in the respective layersafter separation into two layers, whereby the curability of the layercontaining resin (a) can be secured and the simultaneous curing of thelayer containing resin (b) can be realized, with the result that theinterlayer adhesion in the multilayer film can be improved and themultilayer appearance after top coating can further be improved.

[0068] Further, as means for promoting distribution and dissolution theblocked polyisocyanate in the resin layer comprising the particles Acontaining the above resin (a), it is also possible to devise that ablocked polyisocyanate partly having an unblocked isocyanato group bereacted with the hydroxyl group which the above resin (a) contains inadvance so that the resin (a) and curing agent can migrate together onthe occasion of layer separation involving simultaneous curing of thelayer containing resin (a) and the layer containing resin (b).

[0069] The mixing ratio of the above blocked polyisocyanate to the resin(a) may vary according to the degree of crosslinking required for theintended use of the cured coating films, but, in view of the physicalproperties of coating films and the applicability for the top coating,it is preferably within the range of 10 to 50% by weight, on the solidbasis, relative to 100 parts by weight of the resin (a) on the solidbasis. An amount less than 10% by weight will lead to insufficientcuring of coating films, hence decreased physical properties of coatingfilms, such as decreased mechanical strength thereof and, in someinstances, to a bad appearance resulting from coating film erosion bythe thinner of the coating in the step of top coating. An amountexceeding 50% by weight may conversely cause excessive curing, resultingin poor physical properties of coating films, such as poor shockresistance.

[0070] In the electrodeposited coating film formed from theabove-mentioned electrodeposition coating, the resin layer formed fromthe particles B containing the above resin (b) has a dynamic glasstransition temperature of 60 to 150° C. When it is lower than 60° C.,the difference in solubility parameter from that (δa) of resin (a)cannot be made not less than 1.0 but the coating film obtained will bepoor in corrosion resistance. If it is above 150° C., the coating filmobtained will be too hard, allowing cracking in some instances. It ispreferably 80 to 140° C. The above dynamic glass transition temperaturecan be determined according to the method mentioned above.

[0071] From the viewpoint of expression of good rust preventing effectson electroconductive substrates, it is preferred that the above resin(b) be a cation-modified epoxy resin.

[0072] The above cation-modified epoxy resin can be produced byreacting, for ring opening, the epoxy ring in a starting material resinmolecule with an amine such as a primary amine, secondary amine ortertiary amine acid salt. The above starting material resin ispreferably a polyphenol polyglycidyl ether type epoxy resin which is theproduct resulting from the reaction of a polycyclic phenol compound,such as bisphenol A, bisphenol F, bisphenol S, phenol novolak or cresolnovolak, with epichlorohydrin. As examples for other starting materialresins, there may be mentioned those oxazolidone ring-containing epoxyresins which are described in Japanese Kokai Publication Hei-05-306327.These epoxy resins are obtained by reacting, with epichlorohydrin, adiisocyanate compound or a bisurethane compound obtained by blocking theNCO groups of a diisocyanate compound with a lower alcohol such asmethanol or ethanol.

[0073] The above starting material resin can be used after chainextension, prior to epoxy ring opening reaction with an amine, by meansof a bifunctional polyester polyol, polyether polyol, a bisphenol, adibasic carboxylic acid or the like. Similarly, prior to epoxy ringopening with an amine, amonohydroxy compound, such as 2-ethylhexanol,nonylphenol, ethylene glycol mono-2-ethylhexyl ether or propylene glycolmono-2-ethylhexyl ether, may be added partially to the epoxy ring forthe purpose of adjusting the molecular weight or amine equivalent orimproving the thermal flow characteristics.

[0074] As the above amine, there may be mentioned those specificallymentioned hereinabove referring the cationic emulsifier.

[0075] As for the method of introducing a cationic group into the aboveepoxy resin, the production method described in Japanese KokaiPublication Hei-11-209663 which comprises modifying the epoxy ring intoa sulfonium salt is preferred.

[0076] The above cation-modified epoxy resin preferably has a numberaverage molecular weight in the range of 1,500 to 5,000. If it is lessthan 1,500, physical properties such as the solvent resistance andcorrosion resistance of cured coating films may be poor. If it exceeds5,000, it will become difficult to control the resin solution viscosity,hence to synthesize the resin, and the viscosity of the resin obtainedwill become high, hence difficult to handle in the step ofemulsification/dispersion. Furthermore, in some instances, the flowcharacteristics will be poor in the step of heating/curing and thecoating film appearance may be markedly impaired.

[0077] The molecule of the above resin (b) is preferably designed suchthat the hydroxyl value thereof falls within the range of 50 to 250. Ifthe hydroxyl value is less than 50, the curing of coating films willbecome insufficient and, if, conversely, it exceeds 250, excess hydroxylgroups will remain in the coating film after curing, whereby the waterresistance may decrease.

[0078] The particles B containing the resin (b) mentioned above containa curing agent. The above curing agent is not particularly restricted inkind on condition that the resin component can be cured therewith uponheating and it includes those specifically mentioned hereinabove. Amongthem, mention may be made of blocked polyisocyanates, which are suitedfor use as curing agents for electrodeposited resins. The level ofaddition of the above curing agent is the same as mentioned hereinabove.

[0079] The above resin (b), together with the above curing agent, isemulsified/dispersed as such in water to give an emulsion, oremulsified/dispersed in water to give a cationized emulsion by treatmentfor neutralization using a sufficient amount of a neutralizing agent toneutralize the amino groups occurring in each resin. In the step ofemulsion preparation, it is also possible to use the cationic emulsifierspecifically mentioned hereinabove.

[0080] The above method of emulsification/dispersion may be the same asmentioned hereinabove.

[0081] The above electrodeposition coating can be prepared by mixing upthe particles A and particles B obtained in the above manner.

[0082] The mixing ratio between the above resin (a) constitutingparticles A and the above resin (b) constituting particles B ispreferably 5/95 to 70/30 by weight on the solid basis. If it is outsidethe above range, the cured coating film obtained after electrodepositioncoating and baking may not have a multilayer structure; the resin usedin a higher proportion may form a continuous phase while the resin usedin a lower proportion may build up a dispersed phase-forming islandstructure (or microdomain structure). Even if a layer structure isformed, any one of the layers of the multilayer structure will have anextremely diminished thickness, so that any of the shock resistance(chipping resistance) and corrosion resistance will become very poor,hence it is not preferable. A more preferred range is within 10/90 to60/40.

[0083] The resin layer formed from the above particles A preferably hasa dry film thickness of 1 to 20 μm. If it is less than 1 μm, the coatingfilm obtained cannot be expected to be satisfactory in shock absorbingcapacity. If it exceeds 20 μm, the surface roughness will increase,hence the coating film appearance is impaired. More preferred is 3 to 15μm.

[0084] For securing those rust prevention, coating film appearance andhiding power required of the conventional electrodeposited coatingfilms, the resin layer formed by the above particles B preferably has adry film thickness of 5 to 40 μm. If it is less than 5 μm, the corrosionresistance of coating films will be insufficient. If it exceeds 40 μm,the surface roughness will increase and thus the coating film appearancewill be impaired, and the occurrence of coating film defects such asfoaming will become remarkable. More preferred is 10 to 30 μm.

[0085] The above electrodeposition coating generally contains a pigment.

[0086] The above pigment is not particularly restricted but may be anyof those generally used in coatings. Thus, it includes, for example,organic colorpigments such as azochelatepigments, insoluble azopigments, condensed azo pigments, phthalocyanine pigments, indigopigments, perinone pigments, perylene pigments, dioxane pigments,quinacridone pigments, isoindolinone pigments and metal complexpigments; inorganic color pigments such as chrome yellow, yellow ironoxide, red iron oxide, carbon black, titanium dioxide and graphite;extender pigments such as calcium carbonate, barium sulfate, kaolin,aluminum silicate (clay) and talc; and rust preventive pigments such asaluminum phosphomolybdate, lead silicate, lead sulfate, zinc chromateand strontium chromate. Particularly important among them as pigments tobe contained in the cured multilayer film after electrodepositioncoating are carbon black, titanium dioxide, aluminum silicate (clay) andaluminum phosphomolybdate. Titanium dioxide mentioned above is high inhiding power as a color pigment and inexpensive and therefore mostsuited for use in electrodeposited coating films. The above pigments maybe used singly but, generally, a plurality thereof are used according tothe intended purpose.

[0087] The above pigments can be incorporated in the aboveelectrodeposition coating in appropriate amounts after preliminarypreparation of a pigment dispersion paste by dispersing them in acationic pigment-dispersing resin in general use.

[0088] As for the level of addition of the above pigments, the ratio P/Vbetween the whole pigment weight (P) and the weight of all vehiclecomponents other than the pigments (V) in the electrodeposition coatingis preferably within the range of {fraction (1/10)} to ⅓. The term “allvehicle components other than the pigments” mentioned above means thewhole solid components other than the pigments constituting the coating.When the ratio is less than {fraction (1/10)}, the barrier properties ofcoating films against corrosive factors such as moisture may decreaseexcessively due to an insufficient pigment content and, as a result, anypractical level of corrosion resistance may not be expressed. If itexceeds ⅓, a viscosity increase is caused in the step of curing due tothe excessive pigment content, the flow characteristics thus maydeteriorate and the coating film appearance may be markedly impaired.

[0089] In the above electrodeposition coating, there may be incorporatedsuch additives as a rust inhibitor and a surfactant (antifoaming agent)each in an appropriate amount. As the above rust inhibitor which aresoluble in water and easy to use, there may be mentioned, in view of therecent market trend toward exclusion of hazardous heavy metals such aslead, those organic acid salts of zinc, cerium, neodymium, praseodymiumand like rare earth metals. For example, zinc acetate, cerium acetate,neodymium acetate and the like can be incorporated in the aboveparticles B in the step of preparation thereof and added to the coatingin an appropriate amount in a form included or adsorbed in the resinemulsion.

[0090] The above electrodeposition coating is preferably prepared sothat the solid concentration is amount to in the range of 15 to 25% byweight. In adjusting the solid concentration, an aqueous medium, forexample water alone or a mixture of water and a hydrophilic organicsolvent, is used. A small amount of an additive may be incorporated inthe electrodeposition coating. As the additives, there may be mentioned,for example, ultraviolet absorbers, antioxidants, surfactants, coatingfilm surface smoothening agents and curing catalists such as organotincompounds.

[0091] Electrodeposited Coating Film Forming Method

[0092] The method for electrodeposited coating film formation in theabove step (I) comprises the step (1) of applying the aboveelectrodeposition coating to an article to be coated byelectrodeposition coating to thereby obtain an electrodeposited coat andthe step (2) of curing the thus-obtained electrodeposited coat byheating to thereby obtain an electrodeposited multilayer coating film.

[0093] Generally, the electrodeposition coating in the above step (1)can be carried out by connecting an electroconductive substrate, whichis the article to be coated, to a cathode terminal and applying a loadvoltage of 100 to 400 V at a bath temperature of the aboveelectrodeposition coating of 15 to 35° C.

[0094] The electrodeposited coat obtained in the above step (1), byheating in the step (2), undergoes layer separation due to the differentsolubility parameters of the respective resins and gives a curedelectrodeposited film having a multilayer structure such that the layerformed from the particles A occurs on the side contacting with airdirectly and the layer formed from the particles B occurs on the sidedirectly contacting with the article to be coated. The heating in theabove step (2) is generally carried out at 140 to 200° C., preferably160 to 180° C., for 10 to 30 minutes.

[0095] For improving the above layer separation property, preheating maybe carried out following the above step (1). Although the abovepreheating may be conducted at the same temperature as of the heating inthe above step (2), namely carried out successively with the above step(2), it is preferred in the practice of the invention that thepreheating be conducted at a temperature below the curing temperature ofthe electrodeposition coating. By doing so, the layer separationproperty can be improved without deteriorating the coating filmappearance. In that case, the heating temperature may be 60 to 130° C.,and the heating time is about 1 to 10 minutes although it may varyaccording to the heating temperature, etc.

[0096] As for the method of heating in the above steps (1) and (2), thecoated article may be placed in a heater adjusted beforehand to adesired temperature, or the temperature may be raised after placing thecoated article in the heater.

[0097] The article to be coated is not particularly restricted butincludes, for example, iron, copper, aluminum, tin, zinc and othermetals; alloys and castings comprising these metals. Specifically, theremay be mentioned bodies and parts of automobiles such as cars, trucks,motorcycles and buses. More preferably, these metals are subjected inadvance to forming treatment with a phosphate salt, a chromate salt orthe like prior to electrodeposition coating.

[0098] In the above electrodeposition coating, the resin (a) and resin(b) each occurs in an independently emulsified/dispersed state, so thatthe stability of the coating can be secured without any need for givingconsideration to the compatibility between resin (a) and resin (b). If apolar functional group, for example an epoxy group, is introduced intothe resin (a) to secure the compatibility between resin components, asdescribed in japanese Kokai Publication Hei-05-230402, Japanese KokaiPublication Hei-07-207196 and Japanese Kokai Publication Hei-09-208865,there will arise the problem that the elongation percentage andelasticity percentage of the coating films obtained decrease. On thecontrary, the above electrodeposition coating does not require suchmodification but can provide the electrodeposited coating films with ahigh level of shock-absorbing performance characteristics.

[0099] Step (II)

[0100] In the step (II), a water-borne intermediate coating is appliedonto the above electrodeposited coating film formed in the above step(I) to thereby form an uncured intermediate coating film.

[0101] The amount of the volatile basic substance in the above uncuredintermediate coating film is such that the sum of the amount of thevolatile basic substance in the uncured base coating film formed in theabove step (III) is not more than 7×10⁻⁶ mmol per coating film unit area(1 mm²) before carrying out the step (V).

[0102] In the above step (II), the volatile basic substance in thewater-borne intermediate coating remains in the uncured intermediatecoating film formed and, further, in the step (III) to be carried outthereafter, the volatile basic substance in the water-borne base coatingremains in the uncured base coating film formed. When the total amountof these volatile basic substances is in excess of 7×10⁻⁶ mmol percoating film unit area (1 mm²) before carrying out the above step (V)and the above uncured intermediate coating film and the above uncuredbase coating film are heated for curing, the above volatile basicsubstances partly remain and undergo chemical changes and thus causeyellowing. In addition, the basic substances evaporated from the aboveuncured intermediate coating film and the above uncured base coatingfilm are caught up in the inside of the clear coating film formed in theabove step (IV) and cause yellowing in the above clear coating film aswell, hence the color reproducibility or decorativeness of coating filmsare impaired. The total amount of the volatile basic substances in theabove uncured intermediate coating film and the above uncured basecoating film is preferably not more than 6.5×10⁻⁶ mmol per coating filmunit area (1 mm²).

[0103] The above-mentioned “total amount (mmol) of the above volatilebasic substances per unit area (1 mm²)” as so referred to herein meansthe content (mmol) of the above volatile basic substances in thatportion (V) sandwiched between a section (S) and (S′), in which (S) isthe section having an area of 1 mm² on the above uncured base coatingfilm surface and (S′) is the section obtained by projecting the abovesection (S) vertically onto the coated surface of the above article tobe coated. Thus, the sum total [(A+B) mmol] of the amount [A mmol] ofthe volatile basic substance contained in the above uncured intermediatecoating film in the above portion (V) and the amount [B mmol] of thevolatile basic substance contained in the above uncured base coatingfilm in the above portion (V) is the total amount of the above volatilebasic substances per coating film unit area (1 mm²) as so referred toherein.

[0104] The total amount (mmol) of the volatile basic substances per unitarea (1 mm²) mentioned above can be determined by collecting a sample ofthe above uncured intermediate coating film and the above uncured basecoating film, subjecting the sample collected to gas chromatography tothereby determine the content of the above volatile basic substances andmaking a calculation based on the sum of the dry film thickness of theabove intermediate coating film and that of the above base coating filmwhile supposing that 1 gram corresponds to 1 cm³.

[0105] Water-Borne Intermediate Coating

[0106] The water-borne intermediate coating to be used in the above step(II) is one to be applied for the purpose of hiding the substrate,securing the surface smoothness (improving the appearance) after topcoating and providing such coating film physical properties as shockresistance and chipping resistance.

[0107] The above water-borne intermediate coating can be obtained bysolubilizing, dispersing or emulsifying a coating film-forming resinhaving an anionic functional group in a salt form resulting fromcombination thereof with a volatile basic substance as the counter ionin water or a hydrophilic medium such as an alcohol.

[0108] The above coating film-forming resin is not particularlyrestricted but may be any of those resins having an anionic functionalgroup therein, including, for example, acrylic resins, polyester resins,polyurethane resins, alkyd resins and vinyl acetate resins, etc. Amongthem, acrylic resins and polyester resins are preferred in view of thewater resistance and weathering resistance.

[0109] The above anionic functional group is not particularly restrictedbut may be any of those capable of forming a counter ion with thevolatile basic substance, inclusive of carboxyl, sulfonic, phosphoricand like groups.

[0110] These anionic functional groups can be introduced, for example,by producing the above coating film-forming resins by copolymerizationof a monomer component containing a monomer having such an anionicfunctional group. As examples of the carboxyl group-containing monomer,there may be mentioned (meth)acrylic acid, crotonic acid, ethylacrylicacid, propylacrylic acid, isopropylacrylic acid, itaconic acid, maleicanhydride and fumaric acid, etc. Examples of the sulfonicgroup-containing monomer are tert-butylacrylamidosulfonic acid and thelike. The above anionic functional groups may be introduced aftercoating film-forming resin production.

[0111] It is preferred that the above anionic functional groups be atleast partly carboxyl groups.

[0112] The above coating film-forming resin may further contain acationic functional group, such as an amino, imino or hydrazino group,and/or a nonionic functional group, such as a hydroxyl, amido orpolyoxyethylene group, if necessary. The above cationic functional groupand the above nonionic functional group can be introduced by producingthe coating film-forming resin by copolymerization of a monomer havingsuch a functional group and another monomer as desired, and also aftercoating film-forming resin production, these functional groups can beintroduced. Among these functional groups, a nonionic functional groupis preferably contained from the curability viewpoint.

[0113] The above anionic function group-, cationic functional group- andnonionic functional group-containing monomers and the above othermonomers may respectively be used singly or in combination of two ormore.

[0114] The above coating film-forming resin preferably has an acid valueof 5 to 100 (mg KOH/g of resin). If it is less than 5, the adhesion ofcoating films will be poor and the curing may become insufficient. If itis above 100, an excessively high level of hydrophilicity may result,deteriorating the water resistance of coating films. More preferably, itis 30 to 70.

[0115] The above coating film-forming resin, when it is a hydroxylgroup-containing one, preferably has a hydroxyl value of 30 to 150. Ifit is less than 30, the curing maybe insufficient and, if it is above150, excess hydroxyl groups will remain in the coating films aftercuring and may weaken the water resistance. More preferred is 50 to 100.

[0116] The above coating film-forming resin preferably has a numberaverage molecular weight of 1,000 to 30,000. If this is lower than1,000, the cured coating films may become poor in such physicalproperties as solvent resistance and the like. If it is higher than30,000, the resin solution has a high viscosity and becomes difficult tohandle in the procedure for emulsification/dispersion of the resinobtained, and, in addition, the appearance of the coating films obtainedwill be impaired. More preferred is 2,000 to 20,000.

[0117] The method of producing the above coating film-forming resin isnot particularly restricted but may appropriately be selected accordingto the desired species and properties of the above coating film-formingresin, for example from among those production methods known in the artsuch as the molten state polymerization, transesterification,interfacial polymerization and solution polymerization methods.

[0118] The above coating film-forming resin is generally used incombination with a curing agent such as an amino resin and/or a blockedisocyanate resin, etc. In view of pigment dispersibility andworkability, the combination of acrylic resin and/or polyester resinwith a melamine resin is preferred. Such curing agents may be usedsingly or two or more species may be used.

[0119] The above term “volatile basic substance” as used herein means abasic substance having a boiling point not higher than 300° C. The abovevolatile basic substance is used to neutralize the above coatingfilm-forming resin and includes, for example, nitrogen-containing basicsubstances such as inorganic basic substances and organic basicsubstances. The above inorganic basic substances include, for example,ammonia and the like. The above organic basic substances include, forexample, straight or branched C₁₋₂₀ alkyl group-containing primary totertiary amines such as methylamine, dimethylamine, trimethylamine,ethylamine, diethylamine, triethylamine, isopropylamine,diisopropylamine and dimethyldodecylamine; straight or branched C₁₋₂₀hydroxyalkyl group-containing primary to tertiary amines such asmonoethanolamine, diethanolamine and 2-amino-2-methylpropanol; straightor branched C₁₋₂₀ alkyl group- and straight or branched C₁₋₂₀hydroxyalkyl group-containing primary to tertiary amines such asdimethylethanolamine and diethylethanolamine; substituted orunsubstituted C₁₋₂₀ linear polyamines such as diethylenetriamine andtriethylenetetramine; substituted or unsubstituted C₁₋₂₀ cyclicmonoamines such as morpholine, N-methylmorpholine and N-ethylmorpholine;substituted or unsubstituted C₁₋₂₀ cyclic polyamines such as piperazine,N-methylpiperazine, N-ethylpiperazine and N,N-dimethylpiperazine; andlike amines. The above volatile basic substances may be used singly ortwo or more of them may be used.

[0120] Preferred as the above volatile basic substances are amines, morepreferably straight or branched C₁₋₂₀ alkyl group- and/or straight orbranched C₁₋₂₀ hydroxyalkyl group-containing primary to tertiary amines,still more preferably triethylamine and dimethylethanolamine.

[0121] The amount of addition of the above volatile basic substance isnot particularly restricted if sufficient to neutralize the abovecoating film-forming resin. Preferably, however, it corresponds to aneutralization percentage of not more than 120% relative to the resinsolid acid value of the above coating film-forming resin, for instance.At above 120%, the total amount of the above volatile basic substancesin the above uncured intermediate coating film and the above uncuredbase coating film per coating film unit area (1 mm²) cannot be adjustedto 7×10⁻⁶ mmol or below, hence the coating films obtained may turnyellow.

[0122] The use of a water-borne intermediate coating entirely free ofsuch a volatile basic substance as mentioned above in the practice ofthe invention is not excluded if the above-mentioned anionic functionalgroup-containing coating film-forming resin can be stably dispersed.

[0123] The above water-borne intermediate coating generally contains apigment.

[0124] In the above water-borne intermediate coating, those specificallymentioned hereinabove referring to the electrodeposition coating can beused. For the purpose of improving the weathering resistance andsecuring the hiding power, color pigments are preferred. In particular,titanium dioxide is more preferred since it has a white color excellentin hiding power and is inexpensive.

[0125] It is also possible to prepare water-borne standard grayintermediate coatings by using, as the above pigments, carbon black andtitanium dioxide as main pigments, to prepare water-borne set grayintermediate coatings by matching in lightness, hue or the like with thetop coating, or to prepare the so-called water-borne color intermediatecoatings by using various color pigments in combination.

[0126] The above pigments are used preferably in an amount such that theratio of the weight of the pigments relative to the total weight of thepigments and resin solids (PWC) amounts to 10 to 60% by weight in theabove water-borne intermediate coating. At levels below 10% by weight,the pigment amount is insufficient, hence the hiding power may possiblydecrease. At levels higher than 60% by weight, the pigment amount isexcessive, causing an increase in viscosity in the step of curing, hencethe flow characteristics will deteriorate and the coating filmappearance may be impaired.

[0127] In the above water-borne intermediate coating, there may furtherbe incorporated one or more additive components selected from amongultraviolet absorbers, antioxidants, antifoaming agents, surfacemodifiers, foaming inhibitors and so forth.

[0128] The above water-borne intermediate coating can be obtained by anyof the methods known in the art, for example by blending theabove-mentioned coating film-forming resin as it is or one soluble,dispersible or emulsifiable by neutralization with a volatile basicsubstance, if necessary, with a curing agent, a pigment and anotheradditive in a hydrophilic medium. Incases where the above water-borneintermediate coating contains a pigment, it is preferable, from theviewpoint of dispersion stability of the resulting water-borneintermediate coating, to disperse a water-borne pigment paste preparedby dispersing the above pigment using a pigment-dispersing resin in anaqueous medium containing the above coating film-forming resin. Theabove pigment-dispersing resin may be any of those in general use.

[0129] Intermediate Coating Film Forming Method

[0130] The above water-borne intermediate coating is applied onto theelectrodeposited coating film formed in the above step (I) to therebyform an uncured intermediate coating film.

[0131] The method of applying the above water-borne intermediate coatingis not particularly restricted. For example, the coating can be appliedusing an air electrostatic sprayer commonly called “React gun” or arotary atomizer type electrostatic coater commonly called “micro micro(μμ) bell”, “micro (μ) bell” or “meta bell” or the like. The methodusing a rotary atomizer type electrostatic coater is preferred.

[0132] The dry film thickness of the above intermediate coating filmvaries according to the intended use but preferably is 5 to 50 μm. If itis less than 5 μm, the substrate cannot be hidden but film breakage mayoccur. If it exceeds 50 μm, the image sharpness may decrease andtroubles such as unevenness or running may occur in the step ofapplication and, in addition, the content of the volatile basicsubstance in the coating film becomes excessive, hence the total amountof the above volatile basic substances in the above uncured intermediatecoating film and the above uncured base coating film cannot be adjustedto 7×10⁻⁶ mmol or below per coating film unit area (1 mm²) and, in someinstances, the yellowing of cured coating films cannot be satisfactorilyprevented.

[0133] To form uncured coating films using the water-borne intermediatecoating, water-borne base coating and clear coating, respectively, inthe practice of the invention means that the water-borne intermediatecoating, water-borne base coating and clear coating are applied in thatorder by the wet-on-wet technique, and this general idea includes thestep of preheating, for instance. The above step of preheating comprisesallowing the coating film after application to stand or drying the sameat room temperature to about 100° C. for 1 to 10 minutes, for instance.Therefore, the step carrying out preheating can be included prior toapplication of the water-borne base coating and/or application of theclear coating, and such mode of practice constitutes one of theembodiments of the present invention.

[0134] The process involving the above preheating after application ofthe above water-borne intermediate coating but before application of theabove water-borne basecoating promotes the volatilization of thevolatile basic substance in the above uncured intermediate coating film,so that the total amount of the volatile basic substances in the aboveuncured intermediate coating film and the above uncured base coatingfilm can easily be adjusted to 7×10⁻⁶ mmol or less per coating film unitarea (1 mm²) prior to carrying out the above step (V) and the coatingfilms can be prevented from yellowing. Further, the above process givescoating films having a good finish appearance in many instances. Theprocess is thus preferred.

[0135] Step (III)

[0136] In the above step (III), a water-borne base coating is appliedonto the uncured intermediate coating film formed in the above manner tothereby form an uncured base coating film.

[0137] Water-Borne Base Coating

[0138] The above water-borne base coating is applied to provide coatingfilms with beauty and decorativeness, such as color and luster, andmaintain them. It includes, for example, water-borne color basecoatings, water-borne metallic base coatings and water-borne solid basecoatings.

[0139] The above water-borne base coating is not particularly restrictedbut may be any of those which can be obtained by solubilizing,dispersing or emulsifying, in water or a hydrophilic medium such as analcohol, a coating film-forming resin having an anionic functional groupin the form of a salt in combination with a volatile basic substance asthe counter ion. Thus, it includes, for example, those comprising theabove-mentioned coating film-forming resin, a curing agent, a pigmentand other additives.

[0140] Either the above coating film-forming resin or the above volatilebasic substance is not particularly restricted but mention may be madeof, for example, the same ones as described hereinabove referring to thewater-borne intermediate coating.

[0141] The amount of addition of the above volatile basic substance ispreferably not more than 120% expressed in terms of neutralizationpercentage relative to the resin solid acid value of the above coatingfilm-forming resin for the same reasons as mentioned above referring tothe above water-borne intermediate coating.

[0142] In the practice of the invention, the use of a water-borne basecoating entirely free of such a volatile basic substance as mentionedabove is not excluded if the above-mentioned anionic functionalgroup-containing coating film-forming resin can be stably dispersed.

[0143] The above curing agent includes, for example, amino resins and/orblocked isocyanate resins, etc. It is used in combination with the abovecoating film-forming resin. In view of pigment dispersibility andworkability, the combination of acrylic resin and/or polyester resinwith a melamine resin is preferred. Such curing agents may be usedsingly or two or more of them may be used in combination.

[0144] The above water-borne base coating may be used also as a metallicbase coating by incorporating a luster color pigment or as a solid-typebase coating by incorporating a color pigment, for example a red, blueor black, and/or an extender pigment, without incorporating any lustercolor pigment.

[0145] The above luster color pigment is not particularly restricted butincludes, for example, metals, alloys and other colored or uncoloredmetallic lustering materials, and mixtures thereof, interfering micapowders, colored mica powders, white mica powders, graphite or coloredor uncolored flat pigments. Colored or uncolored metallic lusteringmaterials such as metals or alloys, and mixtures thereof are preferredsince they are excellent indispersibility and highly transparent coatingfilms can be formed thereby. Specific examples of the metals arealuminum, aluminum oxide, copper, zinc, iron, nickel, tin and the like.

[0146] The above luster color pigment is not particularly restricted inshape. It may further be colored. For example, it preferably has ascale-like shape with a mean particle diameter (D₅₀) of 2 to 50 μm and athickness of 0.1 to 5 μm. The one having a mean particle diameter withinthe range of 10 to 35 μm is more preferred since it is excellent inluster.

[0147] The pigment concentration (PWC) of the above luster color pigmentin the water-borne base coating is generally not more than 23% byweight. If it exceeds 23% by weight, the coating film appearance will beimpaired. Preferably, it is 0.01 to 20% by weight, more preferably 0.01to 18% by weight.

[0148] Usable as the pigment other than the above luster color pigmentare those color pigments and extender pigments mentioned hereinabovereferring to the electrodeposition coating. One or a combination of twoor more of the luster color pigments, color pigments and extenderpigments can be used as the pigment mentioned above.

[0149] The pigment concentration (PWC) of all the pigments, inclusive ofthe above luster color pigment and other pigments, in the water-bornebase coating is generally 0.1 to 50% by weight, preferably 0.5 to 40% byweight, more preferably 1 to 30% by weight. If it exceeds 50% by weight,the coating film appearance will be impaired.

[0150] As the other additives to be used in the above water-borne basecoating and the method of preparing the above water-borne base coating,there may respectively be mentioned those specifically mentionedhereinabove referring to the water-borne intermediate coating.

[0151] Base Coating Film Forming Method

[0152] The above water-borne base coating is applied onto the uncuredintermediate coating film formed in the above step (II) to thereby forman uncured base coating film.

[0153] As for the above method of application, those methodsspecifically mentioned hereinabove referring to the application of thewater-borne intermediate coating can be mentioned. In cases where theabove water-borne base coating is applied to automotive bodies or thelike, multistage coating, preferably two-stage coating, by using airelectrostatic spray or the combination of using air electrostaticspraying and the above-mentioned rotary atomizer type electrostaticcoater is preferred since, then, the decorativeness can be improved.

[0154] The process for the above-mentioned preheating after applicationof the above water-borne base coating but before application of theabove clear coating promotes the volatilization of the volatile basicsubstance in the above uncured base coating film, so that the totalamount of the volatile basic substances contained in the above uncuredintermediate coating film and the above uncured base coating film caneasily be adjusted to 7×10⁻⁶ mmol or less per coating film unit area (1mm²) and the coating films can be prevented from yellowing. Further, theabove process gives coating films having a good finish appearance inmany instances. The process is thus preferred.

[0155] The dry film thickness of the above base coating film variesaccording to the intended use but preferably is S to 35 μm. If it isless than 5 μm, mottling may occur. If it exceeds 35 μm, the imagesharpness may decrease and troubles such as unevenness or running mayoccur in the step of application and, in some instances, the yellowingof cured coating films cannot be satisfactorily prevented, like in thecase of intermediate coating films mentioned above.

[0156] Step (IV)

[0157] In the above step (IV), a clear coating is applied onto theuncured base coating film formed in the above step (III) to thereby forman uncured clear coating film.

[0158] Clear Coating

[0159] The clear coating is applied for the purpose of smoothing thesurface irregularities, twinkling or the like as caused by the lustercolor pigment when a luster color pigment-containing metallic basecoating is used as the water-borne base coating and also for the purposeof protecting the base coating film.

[0160] The clear coating mentioned above is not particularly restrictedbut may be composed, for example, of a coating film-forming resin, acuring agent and other additives.

[0161] The above coating film-forming resin is not particularlyrestricted but includes, for example, acrylic resins, polyester resins,epoxy resins and urethane resins. These are used in combination with acuring agent such as an amino resin and/or a blocked isocyanate resin.From the viewpoint of transparency or acid etching resistance, the useof a combination of an acrylic resin and/or a polyester resin with anamino resin or the use of an acrylic resin and/or a polyester resinhaving a carboxylic acid-epoxy curing system is preferred.

[0162] The above clear coating, which is applied after the applicationof the above water-borne base coating while it is uncured, preferablycontains a viscosity controlling agent as an additive for the purpose ofpreventing interlayer mingling or inversion or sagging. The level ofaddition of the above viscosity controlling agent is 0.01 to 10 parts byweight, preferably 0.02 to 8 parts by weight, more preferably 0.03 to 6parts by weight, per 100 parts by weight of the resin solids in theclear coating. If it exceeds 10 parts by weight, the appearance will beimpaired. If it is less than 0.1 part by weight, no viscositycontrolling effect will be obtained, hence troubles such as sagging maybe caused.

[0163] The coating form of the above clear coating may be any of anorganic solvent-borne type, water-borne type (aqueous solution, aqueousdispersion, emulsion), non-aqueous dispersion type and powder type. Ifnecessary, a curing catalyst, a surface modifier and the like may beused.

[0164] Clear Coating Film Forming Method

[0165] The above clear coating can be prepared and applied by followingthe conventional method.

[0166] The dry film thickness of the above clear coating film variesaccording to the intended use but preferably is 10 to 70 μm. If the dryfilm thickness exceeds the upper limit, the image sharpness may decreaseand troubles such as unevenness or running and the like may occur in thestep of application and, if it is less than the lower limit, theappearance may be impaired.

[0167] Step (V)

[0168] In the above step (V), the above intermediate coating film, theabove base coating film and the above clear coating film aresimultaneously cured by heating to give a multilayer coating film.

[0169] The above curing by heating is carried out at a temperature of110 to 180° C., preferably 120 to 160° C., whereby cured coating filmswith a high degree of crosslinking can be obtained. At above 180° C.,the coating films will become hard and brittle and, at below 110° C.,curing will be insufficient. While the curing time varies depending onthe curing temperature, 10 to 60 minutes is appropriate in the case ofcuring at 120 to 160° C.

[0170] The multilayer coating film obtained by the method of forming amultilayer coating film according to the invention generally have a filmthickness of 30 to 300 μm, preferably 50 to 250 μm. If it exceeds 300μm, the film physical properties such as thermal shock resistance willdecrease and, if it is less than 30 μm, the strength of the filmsthemselves lowers.

[0171] The electrodeposition coating applied in the above step (I)constitutes a multilayer coating film and thus function division isrealized, so that an electrodeposited coating film simultaneously havinghigh levels of shock resistance (chipping resistance) and corrosionresistance as coating film performance characteristics can be obtained.

[0172] Therefore, multilayer coating films having good corrosionresistance and shock resistance (chipping resistance) and comparable inthese properties to the coating films obtained by the prior artthree-coat three-bake technique comprising curing by heating theconventional electrodeposition coating, intermediate coating and topcoating each time after application of each composition can be obtainedby the so-called three-wet coating comprising applying, onto theelectrodeposited coating film obtained in the above-mentioned step (I),the water-borne intermediate coating, water-borne base coating and clearcoating in the wet-on-wet manner in the above-mentioned steps (II) to(IV) and simultaneously baking these intermediate coating film, basecoating film and clear coating film in the above-mentioned step (V).Furthermore, this three-wet coating makes it possible to omit, from theabove three-coat three-bake technique, that step of baking theintermediate coating which is conventional in the art and thus makes itpossible to construct a novel coating system intended for processcurtailment, cost reduction, energy consumption saving and environmentalload reduction.

[0173] The method of forming a multilayer coating film according to theinvention makes it possible to adjust the total amount of volatile basicsubstances in the above uncured intermediate coating film and the aboveuncured base coating film to not more than 7×10⁻⁶ mmol per coating filmunit area (1 mm²) prior to carrying out the above step (V) and thusprovide finished coating films resistant to yellowing and having a goodappearance.

[0174] In the step of coating film formation from the electrodepositioncoating used in accordance with the invention, multilayerelectrodeposited films can be obtained with a shock-absorbing layerformed on the electrodeposited coating film layer mainly functioning asa corrosion prevention. Furthermore, since the total amount of thevolatile basic substance in the uncured intermediate coating film anduncured base coating film is not more than 7×10⁻⁶ mmol per coating filmunit area (1 mm²), the coating films can be prevented from yellowing.Therefore, the multilayer coating films obtained in accordance with theinvention have good corrosion resistance and shock resistance (chippingresistance) and are comparable in these respects to the conventionalthree-coat films as well as have a good appearance without undergoingyellowing.

[0175] The method of forming a multilayer coating film according to theinvention plays an important role in the coating industry, in particularin the field of automobile coatings, in constructing a novel three-wetcoating system for the purpose of curtailing the baking process,reducing the cost and reducing the environmental load (VOC and HAPs).

BEST MODE FOR CARRYING OUT THE INVENTION

[0176] The following specific examples illustrate the present inventionin detail. They are, however, by no means limitative of the scope of theinvention. “Part (s)” and “%” mean “part (s) by weight” and “% byweight”, respectively.

EXAMPLE 1

[0177] Preparation Example 1 (Preparation of an ElectrodepositionCoating)

[0178] 1-1 (Preparation of a Blocked Polyisocyanate Curing Agent)

[0179] A reaction vessel equipped with a stirrer, nitrogen inlet tube,condenser and thermometer was charged with 222 parts ofisophoronediisocyanate and, after dilution with 50 parts of methylisobutyl ketone, 0.2 part of butyltin laurate was added. After raisingthe temperature to 50° C., 17 parts of methyl ethyl ketoxime was addedin a manner such that the temperature of the contents might not exceed70° C. The mixture was maintained at 70° C. for 1 hour until substantialdisappearance of the isocyanato residue-due absorption was confirmed onan infrared absorption spectrum. Dilution with 10 parts of n-butanolgave the desired blocked polyisocyanate (solubility parameter δi=11.8)with a solid content of 80%.

[0180] 1-2 (Preparation of a Blocked Polyisocyanate Curing Agent)

[0181] A reaction vessel equipped with a stirrer, nitrogen inlet tube,condenser and thermometer was charged with 199 parts of hexamethylenediisocyanate trimer and, after dilution with 39 parts of methyl isobutylketone, 0.2 part of butyltin laurate was added. After raising thetemperature to 50° C., 44 parts of methyl ethyl ketoxime and 87 parts ofethylene glycol mono-2-ethylhexyl ether were added in a manner such thatthe temperature of the contents might not exceed 70° C. The mixture wasmaintained at 70° C. for 1 hour until substantial disappearance of theisocyanato residue-due absorption was confirmed on an infraredabsorption spectrum. Dilution with 43 parts of n-butanol gave thedesired blocked polyisocyanate (solubility parameter δi=10.7) with asolid content of 80%.

[0182] 1-3 (Preparation of a Cation-Modified Epoxy Resin Emulsion[Particles B])

[0183] A reaction vessel equipped with a stirrer, decanter, nitrogeninlet tube, thermometer and dropping funnel was charged with 2,400 partsof a bisphenol A-based epoxy resin (trademark: DER-331J; product of DowChemical) with an epoxy equivalent of 188, together with 141 parts ofmethanol, 168 parts of methyl isobutyl ketone and 0.5 part of dibutyltindilaurate. The mixture was stirred at 40° C. until uniform dissolution,and 320 parts of 2,4-/2,6-tolylene diisocyanate (80/20 weight ratiomixture) was added dropwise over 30 minutes, whereupon heat wasgenerated and the temperature rose to 70° C. Thereto was added 5 partsof N,N-dimethylbenzylamine, and the system inside temperature was raisedto 120° C. and the reaction was allowed to proceed at 120° C. for 3hourswhile distilling off the methanol, until the epoxy equivalent reached to500. Further, 644 parts of methyl isobutyl ketone, 341 parts ofbisphenol A and 413 parts of 2-ethylhexanoic acid were added and, whilemaintaining the system inside temperature at 120° C., the reaction wasallowed to proceed until the epoxy equivalent reached to 1,070, followedby cooling until the system inside temperature lowered to 110° C. Then,a mixture of 241 parts of diethylenetriamine diketimine (methyl isobutylketone solution with a solid content of 73%) and 192 parts ofN-methylethanolamine was added and the reaction was allowed to proceedat 110° C. for 1 hour to give a cation-modified epoxy resin. This resinhad a number average molecular weight of 2,100 and a hydroxyl value of160. Upon infrared absorption spectroscopy and other measurements, theoccurrence of an oxazolidone ring (absorption wave number: 1750 cm⁻¹) inthe resin was confirmed. The solubility parameter was δb=11.4.

[0184] To the thus-obtained cation-modified epoxy resin were added 1,834parts of the blocked polyisocyanate curing agent prepared in the abovePreparation Example 1-1 (mixing ratio of the blocked polyisocyanaterelative to 100 parts by weight of the cation-modified epoxy resin: 38%by weight), 90 parts of acetic acid and, further, 2 parts of zincacetate and 2 parts of cerium acetate as a rust inhibitor. The mixturewas diluted with deionized water to a nonvolatile matter content of 32%and then concentrated under reduced pressure to a nonvolatile mattercontent of 36%. An aqueous emulsion (hereinafter referred to as El)comprising the cation-modified epoxy resin as the main component wasthus obtained.

[0185] 1-4 (Preparation of a Cationic Dispersant for Resin (a))

[0186] A reaction vessel equipped with a stirrer, condenser, decanter,nitrogen inlet tube and thermometer was charged with 114 parts of methylisobutyl ketone and, after heating to 50° C., 75 parts of4,4′-diphenylmethanediisocyanate and 0.1 part of dibutyltin dilaurate asa reaction catalyst were charged and the mixture was heated to andmaintained at 50° C. in a nitrogen atmosphere. Further, 110 parts ofR-15HT (1,4-polybutadiene-α,ω-diol, product of Idemitsu Petrochemical,number average molecular weight=1,200, hydroxyl value=103) was addeddropwise from the dropping funnel over 30 minutes, followed by further30 minutes of stirring. Then, 24 parts of N-methyldiethanolamine, 20parts of ethylene glycol mono-2-ethylhexyl ether and 36 parts of asolution of diethylenetriamine diketimine in methyl isobutyl ketone(solid content 73%) were added, and the reaction was allowed to proceedat 80° C. for 30 minutes. Based on substantial disappearance of theabsorption of the isocyanato group (wave number: 2220 cm⁻¹) on an IRchart for the contents, the reaction was confirmed to be complete. Theresin solution obtained had a solid content of 70%, a number averagemolecular weight of 3, 000 and an amine value of 85.

[0187] 1-5 (Preparation of a Resin Emulsion [Particles A])

[0188] A vessel was charged with 70 parts of R-45HT(1,4-polybutadiene-α,ω-diol, product of Idemitsu Petrochemical, numberaverage molecular weight=2,800, hydroxyl value=47, butadienecontent=99%, solubility parameter δa=9.5), 38 parts of the blockedpolyisocyanate curing agent solution prepared in the above PreparationExample 1-2, 40 parts of the cationic dispersant prepared in PreparationExample 1-4 and 2.5 parts of acetic acid, the mixture was diluted withdeionized water to a nonvolatile matter content of 32% and thenconcentrated under reduced pressure to a nonvolatile matter content of36%. An aqueous emulsion (hereinafter referred to as E2) comprising thecation-modified resin as the main component was obtained.

[0189] 1-6 (Preparation of a Pigment-Dispersing Resin)

[0190] A reaction vessel equipped with a stirrer, condenser, nitrogeninlet tube and thermometer was charged with 710 parts of a bisphenolA-based epoxy resin (trademark: Epon 829, product of Shell Chemical)with an epoxy equivalent of 198 and 289.6 parts of bisphenol A, and thereaction was allowed to proceed at 150 to 160° C. for 1 hour in anitrogen atmosphere. Then, after cooling to 120° C., 406.4 parts of asolution of tolylene diisocyanate half-blocked with 2-ethylhexanol inmethyl isobutyl ketone (solid content 95%) was added. The reactionmixture was maintained at 110 to 120° C. for 1 hour and then 1,584.1parts of ethylene glycol mono-n-butyl ether was added. The mixture wascooled to 85 to 95° C. and homogenized.

[0191] In parallel to the preparation of the above reaction product, aseparate reaction vessel was charged with 384 parts of a solution oftolylene diisocyanate half-blocked with 2-ethylhexanol in methylisobutyl ketone (solid content 95%) and 104.6 parts ofdimethylethanolamine, and the mixture was stirred at 80° C. for 1 hour.Then, 141.1 parts of a 75% aqueous solution of lactic acid was added,47.0 parts of ethylene glycol mono-n-butyl ether was mixed in and themixture was stirred for 30 minutes to give a quaternizing agent (solidcontent 85%). This quaternizing agent (620.46 parts) was added to theabove reaction product, and the mixture was maintained at 85 to 95° C.until the acid value amounted to 1. A pigment-dispersing resin varnish(resin solid content 56%, average molecular weight 2,200) was obtained.

[0192] 1-7 (Preparation of a Pigment Dispersion Paste 1)

[0193] Using a sandmill, a pigment dispersion paste 1 was preparedaccording to the following formulation including the pigment-dispersingresin obtained in Preparation Example 1-6: Pigment-dispersing resinvarnish of Preparation Example 1-6 53.6 parts Titanium dioxide 88.0parts Carbon black 2.0 parts Aluminum phosphomolybdate 10.0 parts

[0194] 1-8 (Preparation of an Electrodeposition Coating)

[0195] An electrodeposition coating (solid concentration 20%) wasprepared using the cation-modified epoxy resin emulsion [particlesB](E1) obtained in Preparation Example 1-3, the resin emulsion [particlesA] (E2) obtained in Preparation Example 1-5, the pigment dispersionpaste 1 obtained in Preparation Example 1-7 and deionized water inrespective amounts such that the mixing ratio of resin (a)/resin (b) (onthe resin solid ratio, without including the curing agent weight incalculation) amounted to 50/50 and the ratio of pigment/resin vehicle(total vehicle weight, including the curing agent weight) (P/V) amountedto ¼.

[0196] In the above electrodeposition coating, an emulsion paste ofdibutyltin oxide as a curing promoter was incorporated in an amount of1.5% as tin amount relative to the amount of solids in the coating.

[0197] Preparation Example 2 (Preparation of a Water-Borne IntermediateCoating)

[0198] 2-1 (Preparation of a Water-Soluble Polyester Resin)

[0199] A reaction vessel was charged with 200.0 parts of isophthalicacid, 179.0 parts of phthalic anhydride, 176.0 parts of adipic acid,150.0 parts of trimethylolpropane, 295.0 parts of neopentyl glycol and2.0 parts of dibutyltin oxide, and the raw materials were melted byheating in a nitrogen gas flow and then the temperature was graduallyraised to 170° C. with stirring. Then, dehydration andtransesterification were effected while raising the temperature furtherto 220° C. over 3 hours. After the acid value became 10, the mixture wascooled to 150° C. Further, 114.0 parts of hexahydrophthalic acid wasadded and the reaction was allowed to proceed for 1 hour and then wasfinished. Further, after cooling to 100° C., 112.0 parts ofbutylcellosolve was added to give a polyester resin. The polyester resinobtained had a solid matter acid value of 50, a hydroxyl value of 65 anda number average molecular weight of 10,000 as determined by GPC.

[0200] The above polyester resin was heated to 60° C., 62 mg or 39 mgper gram of the resin solids of DMEA, respectively, was added, anddeionized water and butylcellosolve were added to a nonvolatile mattercontent of 50%. Water-soluble resin 1 and 2 were obtained.

[0201] 2-2 (Preparation of Pigment Dispersion Paste 2)

[0202] Disperbyk 190 (product of Byk Chemie Japan) (8 parts) as apigment-dispersing resin, 29parts of deionized water, 31 parts ofrutile-form titanium dioxide, 31 parts of barium sulfate and 1 part oftalc were preliminarily mixed together and then mixed up and dispersedat room temperature for 1 hour in a paint conditioner with glass beadmedia added, to give a pigment dispersion paste 2 with a particle sizenot larger than 5 μm.

[0203] 2-3 (Preparation of a Water-Borne Intermediate Coating)

[0204] A water-borne intermediate coating was prepared by formulatingthe water-soluble polyester resin obtained in Preparation Example 2-1and the pigment dispersion paste 2 obtained in Preparation Example 2-2as well as hexamethoxymethylolmelamine (HMMM, tradename: Cymel 235,product of Mitsui Cytec) as a melamine resin in the solid matterproportions shown in Table 1.

[0205] Preparation Example 3 (Preparation of a Water-Borne Base Coating)

[0206] 3-1 (Preparation of a Water-Soluble Acrylic Resin)

[0207] A reaction vessel was charged with 23.9 parts of dipropyleneglycol methyl ether and 16.1 parts of propylene glycol methyl ether, andthe temperature was raised to 120° C. with stirring in a nitrogen gasflow. Then, a mixed solution composed of 54.5 parts of ethyl acrylate,12.5 parts of methyl methacrylate, 14.7 parts of 2-hydroxyethylacrylate, 10.0 parts of styrene and 8.5 parts of methacrylic acid and aninitiator solution composed of 10.0 parts of dipropylene glycol methylether and 2.0 parts of tert-butyl peroxy-2-ethylhexanoate were addeddropwise in parallel to the reaction vessel over 3 hours. Aftercompletion of the dripping, maturation was effected at the sametemperature for 0.5 hour.

[0208] Further, an initiator solution composed of 5.0 parts ofdipropylene glycol methyl ether and 0.3 part of tert-butylperoxy-2-ethylhexanoate were added dropwise to the reaction vessel over0.5 hour. After completion of the dripping, maturation was effected atthe same temperature for 1 hour.

[0209] Then, using a solvent-removing apparatus, 16.1 parts of thesolvent was distilled off under reduced pressure (70 Torr) at 110° C.,22 mg per gram of the resin solids of dimethyl-ethanolamine (DMEA) anddeionized water were added to give a water-soluble acrylic resin with anonvolatile matter content of 31%, a solid matter acid value of 56, ahydroxyl value of 70 and a number average molecular weight of 10,000 asdetermined by GPC.

[0210] 3-2 (Preparation of a Pigment Dispersion Paste 3)

[0211] A pigment dispersion paste 3 with a particle size of not largerthan 5 μm was obtained in the same manner as in Preparation Example 2-2except that 100.0 parts of the water-soluble acrylic resin obtained inPreparation Example 3-1, 28.9 parts of deionized water, 0.3 part ofdimethylaminoethanol and 5.1 parts of Degussa Carbon FW-285 (Product ofDegussa A G) were used.

[0212] 3-3 (Preparation of a Water-Borne Base Coating)

[0213] A water-borne base coating was obtained by formulating 118.8parts of the water-soluble acrylic resin obtained in Preparation Example3-1, 134.3 parts of the pigment dispersion paste obtained in PreparationExample 3-2, 29.1 parts of Cymel 204 (product of Mitsui Cytec) as amelamine resin, and 161.3 parts of deionized water.

[0214] (Application Method)

[0215] The electrodeposition coating obtained in Preparation Example 1was applied by electrodeposition to zinc phosphate-treated dull steelpanels at a voltage such that a dry film thickness of 30 μm wasobtained. The coatings were first preheated at 100° C. for 5 minutes andthen further baked at 160° C. for 15 minutes.

[0216] The water-borne intermediate coating obtained in PreparationExample 2 was applied to the above panels to a dry film thickness of 20μm by the air spray coating method, followed by 5 minutes of preheatingat 80° C. Then, the water-borne base coating obtained in PreparationExample 3 was applied thereto to a dry film thickness of 18 μm by theair spray coating method, followed by 5 minutes of preheating at 80° C.Furthermore, Macflow-O-1801 W Clear (clear coating produced by NipponPaint) was applied thereto to a dry film thickness of 35 μm by the airspray coating method, followed by 60 minutes baking at 180° C. to give amultilayer coating film. TABLE 1 Example Compar. 1 2 Ex. Obtained inPrep. 1 Electrodeposition coating Ex. 1 U-50 Intermediate Water-soluble70 — 70 coating polyester resin 1 Water-soluble — 70 — polyester resin 2HMMM 30 30 30 Pigment 100 100 100 dispersion paste 2 Chipping resistance4 4 1 Amount of Intermediate 3.8 2.4 3.8 residual DMEA coating film[(A)] Base coating film [(B)] 2.2 2.2 2.2 [(A) + (B)] 6.0 4.6 6.0Yellowing b value after 5.6 5.6 5.6 baking (180° C. × 60 min.) Visual ⊚⊚ ⊚ observation level

[0217] (Evaluation Methods)

[0218] (1) Evaluation of the Electrodeposition Coating

[0219] The evaluation results of properties and performancecharacteristics of the respective electrodeposited coating filmsobtained are shown in Table 2.

[0220] In Table 2, the layer directly contacting air is referred as“upper layer” and the layer directly contacting the conductive substrateas “lower layer” for convenience' sake.

[0221] (1-1) Stability of the Coating

[0222] The electrodeposition coating was maintained at 30° C. andstirred for 1 month. Thereafter, one liter thereof was filtered through400-mesh wire gauze. The residual solid matter on the wire gauze weighedless than 5 mg, the stability was judged as good.

[0223] (1-2) Layer Separation in the Electrodeposited Coating Film

[0224] The section was visually observed using a video microscope. Incase a multilayer electrodeposited coating film was found, the mainresin constituting each layer was identified by FTIR-ATR analysis.

[0225] (1-3) Thickness of Each Layer

[0226] The thickness was measured based on the results of thecross-section observation using the video microscope mentioned above.

[0227] (1-4) Elongation Percentage of the Upper Layer-Forming Resin

[0228] Specimens for tensile testing were separately prepared accordingto JIS K 6301 using the resin (a)-containing emulsion obtained inPreparation Example 1-5 and measured. The curing conditions were thesame as the coating film curing conditions mentioned above.

[0229] (1-5) Tg (Dynamic Glass Transition Temperature) of the UpperLayer and of the Lower Layer

[0230] A multilayer electrodeposited coating film formed on a tin platewas peeled off using mercury and cut to prepare specimens formeasurements. The specimens were once frozen using liquid nitrogen on aRheometrics dynamic analyzer RDA-II tester (product of Rheometrics, USA)and then given vibrations with a frequency of 10 Hz at a rate oftemperature rising of 2° C. per minute and measured for viscoelasticity.The ratio (tanδ) of the loss elastic modulus (E″) relative to thestorage elastic modulus (E′) was calculated and each dynamic Tg wasdetermined by determining the point of inflexion thereof.

[0231] (1-6) Electrodeposited Film Surface Roughness

[0232] The coated panel obtained was measured for surface roughness Raaccording to JIS B 0601 using Handy Surf E-30A (product of TokyoSeimitsu) (cutoff 0.8 mm).

[0233] (1-7) SDT

[0234] The coated panel was given cross cuts reaching the substrate bymeans of a knife and subjected to salt solution immersion testing (5%aqueous solution of sodium chloride, 55° C.) for 240 hours and peelingwas attempted from both sides of the cut area using an adhesive tape.The maximum width of the portion peeled off was indicated.

[0235] (1-8) SST

[0236] The coated panel was given cross cuts reaching the substrate bymeans of a knife and subjected to salt spray testing (5% aqueoussolution of sodium chloride) for 240 hours. The maximum width of rustgenerated from the cross-cut portion was indicated.

[0237] (1-9) Shock Resistance

[0238] Using a DuPont impact tester, a weight of 1 kg was allowed tofall upon the panel from a height of 50 cm at room temperature and thepanel was examined for coating film breaking and peeling. TABLE 2Preparation Electrodeposition coating Example 1 U-50 Stability of thecoating Good Good Electrodeposited film Separation into Unilayer sectionobservation two layers structure Layer thickness A (μm) 15 — Layerthickness B (μm) 15 — Elongation (%) of upper 800 — layer-forming resinTg (° C.) of upper layer −50 — Tg (° C.) of lower layer 90Electrodeposited film 0.15 0.15 surface roughness Ra SDT 0.1 mm 0.1 mmSST 1.0 mm 1.0 mm Shock resistance No breaking, Breaking and no peelingpeeling

[0239] (2) Amount of Residual DMEA in the Intermediate Coating Film andBase Coating Film

[0240] Samples of the uncured intermediate coating film and of theuncured base coating film were collected following the preheatingrespectively after application of the water-borne intermediate coatingand after application of the water-borne base coating by theabove-mentioned method of application. A 0.3-g portion of each samplecollected was weighed, 0.6 ml of a standard solution (100 ml of methanoladmixed with 1.0 g of isobutanol) was added, the mixture was thoroughlystirred with a touch mixer and by ultrasonic vibrations and thencentrifuged.

[0241] The supernatant was collected and assayed for DMEA amount by gaschromatography. Based on the amount of DMEA thus obtained and the sum ofthe dry film thickness of the uncured intermediate coating film and thedry film thickness of the uncured base coating film, the total amount ofDMEA per coating film unit area (1 mm²) was calculated, supposing that 1g corresponds to 1 cm³. The results are shown in Table 1.

[0242] (3) Evaluation of the Multilayer Coating Films Obtained

[0243] (3-1)Chipping Resistance

[0244] The coated panel cooled to −30° C. was mounted on the specimenholder of a flying stone testing machine (product of Suga Shikenki) sothat the stone might collide with the panel at an angle of 90°. The No.7 crushed stone weighing 100 g was shot at an air pressure of 3 kg/cm²and caused to collide with the coated panel. The extent of chipping(number, size, site of breakage) was evaluated on the 5-score scale. Theresults are shown in Table 1.

[0245] 1: Chipping all over the surface, peeling from the substrate.

[0246] 2: Chipping all over the surface, no peeling from the substrate.

[0247] 3: Partial chipping, peeling from the substrate.

[0248] 4: Partial chipping, no peeling from the substrate.

[0249] 5: Substantially no breakage.

[0250] (3-2) Yellowing

[0251] The b value of the coating films after 60 minutes baking at 180°C. was measured using a color difference meter (SM Color Computer SM-4,product of Suga Shikenki). The higher the b value is, the stronger theyellowness is. The results are shown in Table 1.

[0252] The yellowness was further evaluated by visual observationaccording to the following criteria:

[0253] ⊚: No yellowing is observed at all.

[0254] ◯: Little yellowing is observed.

[0255] X: Yellowing is observed.

EXAMPLE 2

[0256] Coated panels with a multilayer coating film formed thereon wereproduced and evaluated in the same manner as in Example 1 except thatthe water-borne intermediate coating was prepared according to theformulation indicated in Table 1. The results are shown in Table 1.

COMPARATIVE EXAMPLE 1

[0257] Coated panels with a multilayer coating film formed thereon wereproduced and evaluated in the same manner as in Example 1 except thatPowertop U-50 (cationic electrodeposition coating, product of NipponPaint) was used as the electrodeposition coating. The results are shownin Table 1 and Table 2.

[0258] As shown in Table 2, it was found that while the conventionalelectrodeposition coating forms a single-layer coating film, theelectrodeposition coatings to be used according to the invention formmultilayer electrodeposited coating films of high resin elongationpercentage, with the lower layer having a higher Tg and the upper layerhaving a lower Tg. As is seen from Table 1, it was found that themultilayer coating films obtained in Examples are superior in chippingresistance and, when the total amount of residual DMEA per unit area (1mm²) of the uncured intermediate coating film and uncured base coatingfilm is not more than 7×10⁻⁶ mmol, almost no yellowing of the coatingfilms is observed either in terms of b value or upon visual observation.

1. A method of forming a multilayer coating film comprising the step (I)of coating an article to be coated with an electrodeposition coatingfollowed by curing by heating to form an electrodeposited coating film,the step (II) of applying a water-borne intermediate coating onto saidelectrodeposited coating film to form an uncured intermediate coatingfilm, the step (III) of applying a water-borne base coating onto saidintermediate coating film to form an uncured base coating film, the step(IV) of applying a clear coating onto said base coating film to form anuncured clear coating film and the step (V) of curing said intermediatecoating film, said base coating film and said clear coating filmsimultaneously by heating to thereby obtain a multilayer coating film,wherein said electrodeposition coating contains a particle A containinga resin (a) whose solubility parameter is δa as well as a particle Bcontaining a curing agent and a resin (b) whose solubility parameter isδb and satisfies that (1) the value of (δb-δa) is not less than 1.0, (2)as regards the electrodeposited coating film formed from saidelectrodeposition coating, the resin film formed from said particle Ashows a dynamic glass transition temperature of −110 to 10° C. and thecoating film obtained by film formation from said particle A alone showsan elongation percentage of not less than 200% and (3) as regards theelectrodeposited coating film formed from said electrodepositioncoating, the resin film formed from said particle B shows a dynamicglass transition temperature of 60 to 150° C., and wherein the totalamount of volatile basic substance in said uncured intermediate coatingfilm and said uncured base coating film prior to carrying out the step(V) is not more than 7×10⁻⁶ mmol per coating film unit area (1 mm²). 2.The method of forming a multilayer coating film according to claim 1,wherein the particle A contain a curing agent and solubility parameterδi of at least one of said curing agent satisfies δb>δ>δa.
 3. The methodof forming a multilayer coating film according to claim 1, wherein theweight ratio between the resin (a) and the resin (b) on the solid basisis 5/95 to 70/30.
 4. The method of forming a multilayer coating filmaccording to claim 1, wherein the resin (a) is obtained bypolymerization of a monomer component comprising at least 50% by weightof a conjugated diene monomer.
 5. The method of forming a multilayercoating film according to claim 1, wherein the step (II) comprisescarrying out preheating after application of the water-borneintermediate coating.
 6. The method of forming a multilayer coating filmaccording to claim 1, wherein the step (III) comprises carrying outpreheating after application of the water-borne base coating.
 7. Amultilayer coating film which is obtained by the method of forming amultilayer coating film according to claim 1.